Post on 24-Feb-2016
description
SPACE-TIME PICTURE OF HADRONIC AND NUCLEAR COLLISIONS
From low to high energies
2
COLLISION @ ALICE16.05.2008
3
COLLISION @ ALICE16.05.2008
Higgs?
junk?
It is essential to understand hadronic interactions at LHC!
the structure of the underlying event
behaviour of σtot, σel and σdiff with energy...
99,9999% of event!! pp interactions at LHC are
small AA systems?
...or else, no discovery!
4
OVERVIEW What is a hadron/nucleus? What are
the relevant degrees of freedom? How does a hadron interact with
another hadron? What about nuclei? What is the difference of these
interactions going from low to high energies?
16.05.2008
7
hadrons, nuclei
16.05.2008
COMPLEX BUILDING BLOCKS
8
WHAT IS A HADRON? strongly interacting
composite subatomic particle consists of quarks
baryons: 3 quarks, fermion mesons: quark – anti-quark
pair, boson excited states - resonances at high enough temperatures
they dissolve!
16.05.2008
u
d
u
du
u d
9
STRANGE AND STRONG INTERACTION16.05.2008
q q
qq qq
Quantum ChromoDynamics (QCD) – fundamentalo calculable for short distance processeso calculable on lattice for static situationso useless for large distance physics, dynamics...
need approximations, models, ideas!
10
NUCLEAR PHYSICS AT HIGH ENERGIES
”macroscopic” volumes conditions ~early Universe free quarks and gluons? Quark-Gluon Plasma (QGP)
16.05.2008
u
du
11
HYDRODYNAMICAL PICTURE
need to assume infinitesimally small cells are locally thermalized large rate of interaction
can define temperature, energy density and pressure
16.05.2008
Lev Landau
12
THERMALIZATION
usually: particles start with an arbitrary velocity distribution
”equilibrate” over time, maximization of entropy take the continuum limit – loosing the concept
of ”particles”
16.05.2008
0 T
pguupT
13
BJORKEN INITIAL CONDITION
boost invariant initial condition, adiabatical expansion
the longitudinal motion is uniform: vz=z/t
gives rise to a central plateau – height independent of energy!
initial energy density
16.05.2008
dydE
RT
Bj0
2
11
14
PICTURE OF A FAST HADRON
16.05.2008
15
FEYNMANS (NAIVE) PARTON MODEL
• in a frame where the hadron is moving infinitely fast, it consists of infinitely many partons
• the partons carry a fraction of the total momentum of the hadron each
• the partons are free!
16.05.2008
ud
u
16
HOW CAN THIS BE?
uncertainty principle! the constituent quarks can
interact via gluon exchange fluctuate into a quark-antiquark pair
when boosting the proton, the timescales related to these fluctuations are Lorentz dilated interactions of quarks now take place over much
larger timescales..
16.05.2008
18
DISTRIBUTION OF QUARKS AND GLUONS
the probatility of finding a parton (quark, gluon, heavy quark...) with momentum fraction x of the total proton
non-perturbative quantity but dependence on
”resolution” given by QCD
universal quantity measured in ep
collisions can predict νp, pp
etc...
16.05.2008
heavy-ion collisions at RHIC!
22
PICTURE OF A FAST HADRON16.05.2008
fast partons
slow partons
target
22
12222
mM2ppmpM
ΔE1τ
nxxx ...1 10
1t 2t 3t< <
The lifetime of the fluctuation can be quite large!
Strong ordering from fast to slow partons:
Only correlation between ”nearest neighbours”...
V.N. Gribov
24
TRANSVERSE SPACE PICTURE
random walk in the transverse plane each step ~1/μ # of steps ~ln(E)
interaction range
16.05.2008
R
initial parton
target
22 )ln(
ER
“OCTOPUS” PICTURE
yybP y
tsf ee'41),( '42
25
PICTURE OF A FAST HADRON16.05.2008
p
Rlog
2|| pR
fast partons- localized in a contracted disc
slow partons- spread out on long itudinal distances
xpplL0
11
soft part of the fast hadron is not Lorentz contracted!AT HIGH ENERGY THE PROTON CAN BECOME LARGER THAN A NUCLEUS!
mostly gluons at low-x:
26
FROM A NUCLEUS’ POINT OF VIEW...16.05.2008
overlap of nucleons in the nucleus huge density of gluons
27
hadron-hadron, hadron-nucleus, nucleus-nucleus
16.05.2008
COLLISIONS
28
COMPLICATED PROBLEM
in principle an infinite series need to find the most important terms
single-Pomeron diagram describes existing data quite OK! problems with the theory at high energies,
eg. LHC! need multi-Pomeron exchanges
16.05.2008
...++ ...+
Donnachie, Landshof
29
MULTIPERIPHERAL MODEL
the reggeon can be seen as an exchanged ladder of particles in the t-channel
the partons take a fraction ε of the initial energy
#of particles in the fluctuation: n~ln(E) Reggeon is a highly non-local object
16.05.2008
Amati, Fubini, StanghelliniGribov
30
IN THE CENTER OF MASS FRAME16.05.2008
single-ladder exchange a large amount of additional particles are created! Feynman-plateau
projectile
target
yy
”central plateu”FIN
AL STATE
32
MULTIPLE SCATTERING
1st correction to the single-ladder exchange
”classical” rescattering picture low and intermediate energies –
potential scattering
16.05.2008
33
THE GLAUBER MODEL16.05.2008
heavy-ion collisions in each rescattering there is a
certain probability for particle production
34
THE GLAUBER MODEL16.05.2008
2RA
2RA/γ
# of collisions
xxnxn
xnxddsd
collpart
pp 2)1(
2
3
entropy density related to number of interactions
”initial condition”
35
16.05.2008
BUT IS THIS PICTURE VALID AT HIGH ENERGIES?
36
MULTIPLE LADDER STRUCTURE
at high energies, multiple ladders can start to evolve simultaneously
gives rise to novel physical phenomena
16.05.2008
37
NON-PLANAR GRAPHS
planar diagram vanishes at high energies ladders require a long time to develop critical energy E0=mNμRA
the projectile goes into a fluctuation long before the collision takes place ladders develop at the same time
16.05.2008
MandelstamGribov
38
COHERENT INTERACTION16.05.2008
the projectile becomes large compared to the target
interacts simultaneously with the whole system effectively less interaction - shadowing dramatic change of space-time picture
39
CHANGE OF SPACE-TIME PICTURE16.05.2008
the diagrams corresponding to ”classical” rescatterings are suppressed at high energies!
Gribov trick: Glauber is OK after all! Almost... have to take into account diffractive
intermediate states!
E1
(2)totσ + + . . .totσ
42
planar diagrams
non-planar
SMOOTH BEHAVIOUR OF OBSERVABLES
the observables don’t feel the change of space-time picture
at high energy: nuclear shadowing are there any observables that are
sensitive to this transition?
16.05.2008
E
|Δσ2|
AN RmE 0
totalσdiffractive
43
A1/3Enhanced in AA collisions
CASCADING – ENHANCED DIAGRAMS16.05.2008
k
All particles with can interact. Ο(R)k z
=Triple-Pomeron coupling:quite small or quite large?important at high energiesparton saturation!
45
WHAT HAPPENS AT LHC?
interesting physics! probing the strongest force in new
domain! evolution of partons resembles
growth of bacteria colony reaction-diffusion process
striking experimental features!
16.05.2008
46
OUTLOOK16.05.2008
will large interaction of partons early on prepare a thermalized system in AA? in pp?
47
16.05.2008
48
COHERENCE CORRECTIONS
σtot=σ(hp)+σ(hn)=2(σinel+σel)=2Sξ+Sξ2/2
impulse approximation result
16.05.2008
Corrections: loss of flux in the second interaction -Sξ2
contrib to double multiplicity event -Sξ2
contrib to double multiplicity event Sξ2
enhancement of elastic cross section Sξ2/2 12-4
AGK ratios
Bartels, Ryskin Z. Phys. C 76, 241
49
16.05.2008
22
e)0(1'21
yigxy
L
50
16.05.2008
mpln)ln( AmR
#par
ticle
s
y
hadron targetnucleus target
51
16.05.2008
absorption
shadowingE
correction factor
AN RmE 0
52
STAGES OF A HEAVY-ION COLLISION16.05.2008
Nuclear GeometryParton distributionsNuclear shadowing
Parton production& reinteraction
Chemical Freeze out &Quark Recombination
Jet FragmentationFunctions
Hadron Rescattering
Thermal Freeze out &Hadron decays
0 fm/c
2 fm/c
7 fm/c
>7 fm/c